1. Field Of The Invention
The invention pertains to novel aqueous acid compositions comprising N-hydroxyethyl ethylenediamine triacetic acid (HEDTA), and another organic acid selected from the group consisting of water soluble aliphatic monocarboxylic, aliphatic polycarboxylic, and aliphatic hydroxylated mono- or polycarboxylic acid. Preferably an acid corrosion inhibitor is also present. The invention also pertains to a method of using such compositions to chemically clean (remove) iron oxide scale from metal surfaces and a method of passivating the clean surface against corrosion. 2. Technology Review:
The invention utilizes an organic polycarboxylic acid referred to as N-hydroxyethyl ethylenediamine triacetic acid (abbreviated as HEDTA). This known compound and a method of using it to chemically clean iron oxide scale from metal surfaces is fully disclosed in our U.S. Patent 4,430,128.
HEDTA has also been used as a chelant. Ammoniated or aminated salts of HEDTA have been used as chelants in removing the scale from metal surfaces and for passivating ferrous metal surfaces. The salts were said to be effective against water hardness type scale (i.e. predominantly calcium and/or magnesium salts, such as calcium sulfate, calcium carbonate, etc.) and scales containing a high iron oxide content. However, when ammoniated or aminated salts of HEDTA have been used for chemical cleaning, the pH has generally been weakly acidic or basic, preferably basic. The use of ammoniated ethylenediamine tetraacetric acid (abbreviated as EDTA) to clean and passivate metal surfaces is described in U.S. Pat. No. 3,308,065, No. 3,413,160, and No. 3,438,811. EDTA was normally used as the ammoniated salt, or the sodium salt, at a pH from about 8.5 to about 10.